The present invention pertains generally to magnetic tape drives, and more particularly to a method and apparatus for preventing tape slack between the capstan and takeup reel in a tape drive by adaptively controlling the tape tension between the capstan and takeup reel.
Tape storage technology is routinely used for routine system back up and long-term data archiving. Conventional tape storage devices rely on some form of tape motion control to transport the tape across a magnetic head as it moves between the supply reel and take-up reel of the tape drive.
The most common form of tape motion control is accomplished via a capstan assembly. In this type of system, the tape is pinched between a rotating capstan and a pinch roller during normal read/write operation. The force of the pinch roller against the portion of the tape contacting the capstan causes enough frictional force to move the tape as the capstan rotates. During a normal read/write operation, a takeup reel motor is also engaged to rotate the takeup reel in a direction to receive the tape passing across the capstan from the supply reel. The tension of the tape in the tape path between the capstan and takeup reel is affected by the rotational speed of the capstan in relation to the rotational speed of the takeup reel.
Tape tension control is achieved by monitoring both capstan tachometers and reel tachometers. Reel tachometers provide speed feedback information for each reel, which is used by the tape drive controller to control the reel speed. Reel motor voltage/current measurements are monitored by the controller and used to control the reel motor voltage/current in order to control the tape tension between the capstan and takeup reel during forward movement of the tape and between the capstan and supply reel during backward movement of the tape. The capstan tachometer provides speed information about the capstan. It is used by tape controller to verify proper direction and speed control of the capstan motor.
The tape tension between the capstan and reels can be derived with knowledge of the relationship between the amount of tape on each of the supply and takeup reels. Because the amount of tape on each of the supply and takeup reels continuously changes as the tape moves from one reel to the other, the radius of the amount of tape on the supply and takeup reels must be continuously calculated. The radii calculations are based upon the capstan and reel tachs. Because the relationship between a given reel tach and the position of the tape is not linear (due to the non-linear relationship between the radius of tape on a reel and the linear position of the tape), the reel radii calculations are also nonlinear. This calculation is typically handled by reel tension control circuitry. During manufacturing, the values of the reel motor voltage/currents for the calibrated tensions are set, typically by storing them in non-volatile storage. When the drive is running, the reel tension control circuitry outputs a voltage control signal to the reel motors. The voltage control signal changes based upon the radii and the measured amplifier voltage.
Tape slack can occur for a variety of reasons. Generally, the reel motor voltage/current is calibrated at the time of manufacture to a fixed voltage/current level. As the tape drive ages, the calibrated reel motor voltage/current value often becomes inadequate due to changes in tape binding, temperature, or friction.
Prior art methods of maintaining correct tension between the capstan and reels as the reel radii vary and as the drive temperature or power supply voltages fluctuate have required active tension sensors and dual reel motors or slip clutches in the supply and takeup hubs. Both techniques are costly in terms of occupying valuable circuit board space, and requiring additional components, additional circuit complexity, and additional drive assembly and test time. In addition, slip clutches tend to provide inconsistent tension from unit to unit and over the life of the tape drive.
Accordingly, a need exists for a more precise and less costly technique for controlling the tape tension between the capstan and tape reels. It would be desirable that this technique operated consistently from tape drive unit to tape drive unit and over temperature and voltage variations. Furthermore, it would be desirable to have such a technique without requiring additional components.
The present invention is a novel method and apparatus for precisely controlling the tape tension between the capstan and tape reels. In particular, the present invention allows tape tension control that is consistent from drive to drive and over varying temperature and manufacturing process variations. Furthermore, the present invention allows precise control without requiring additional components that add to cost, complexity, and test time.
In accordance with the invention, in a tape drive mechanism that actuates the reel hubs directly from a single reel motor via a gear train, the tension of the tape between the capstan and tape reels is adjusted by creating a phase locked loop (PLL) between the capstan tachometer pulses and the reel hub tachometer pulses. The amount of voltage applied to the reel motor is slaved to the capstan tachometer frequency. This allows the reel motor voltage/current to be adaptively adjusted to maintain proper tape tension between the capstan and tape reels. Because the efficient and novel tape tensioning technique of the invention requires fewer components than prior art solutions of the tape tensioning problem, the invention allows a lower cost tape drive that is more immune from the aging effects of the system components.